This invention concerns a method for cleaning the SiO2 grain, by means of heating the SiO2 grain comprising contaminations to a temperature at which the contaminations soften or form melting agglomerates with the SiO2 grain, and the separation of contaminations and SiO2 grain. Furthermore, the invention concerns a device for the implementation of the method.
DE-C1 33 21 589 describes a generic method for the cleaning of quartz sand and a device for the implementation of the method. For the separation of mineral contaminations in quartz sand, present for example as intergrowth of feldspar or garnet with quartz grains, it is proposed to heat a screen fraction of the quartz sand, sized 180 xcexcm to 250 xcexcm, to a temperature of 1,370xc2x0 C. in an electrically heated rotary furnace with an SiC rotary tube. Due to this heat treatment performed over a period of 30 minutes, the contaminations soften so as to form melting agglomerates with each other or with quartz grains. In contrast, the quartz grains themselves do not soften so that they will essentially retain their original size and morphology. After cooling down, the melting agglomerates are screened out or separated from the purified quartz sand by means of air sifting.
This method enables a batch-wise cleaning of quartz sand from contaminations, under the prerequisites that the contaminations will bind in melting agglomerates and that the melting agglomerates are larger than the employed screen fraction of the quartz grain. It has been shown, however, that these prerequisites are not always met and that, moreover, the melting agglomerates are mechanically unstable and easily disintegrate again during the separation process and thus cannot be easily removed from quartz sand by means of screening or air sifting.
For many applications of SiO2 powderxe2x80x94for example as the starting material for quartz glass components to be used in semiconductor manufacture or for optics, the starting materials"" purity is subject to extremely high requirements which can be met by the known method only with great expenditures of time, materials and costs. To avoid any contaminations due to abrasion during the cleaning process, high-purity, partly high-temperature resistant, expensive device components are requiredxe2x80x94such as rotary tubes made of SiC, for example.
Some of these disadvantages are avoided by a cleaning method suitable for the continuous cleaning of quartz powder, as described in EP-A1 737 653. The quartz powder to be cleaned, with a mean grain size between 106 xcexcm and 250 xcexcm, is continuously fed to an electrically heated rotary furnace of quartz glass in which it runs successively through a preheating chamber, a reaction chamber and a gas desorption chamber. In the preheating chamber, the quartz powder is heated to approx. 800xc2x0 C., and subsequently treated in the reaction chamber at a temperature of about 1,300xc2x0 C. with a gas mixture of chlorine and hydrogen chloride. The quartz powder""s alkali and alkaline earth contaminations will react with the chloric gas mixture, forming gaseous metal chlorides. The treatment gas and the gaseous reaction products are subsequently exhausted.
In this manner, especially such contaminations can be removed which can pass over into the gas phase through hot chlorination. The known method thus achieves a significant reduction of alkali and alkaline earth contaminations in the quartz powder. However, the method is unsuitable for contaminations which cannot be removed by chlorination. Moreover, the degree of purification depends on the quartz powder""s reaction period with the chloric gas mixture and on the reaction temperature. At higher temperatures, chlorine reacts faster with the metallic contaminations so that a better cleaning effect could be expected with increasing temperatures. However, the softened grain tends to form agglomerates which impedes free access of the treatment gas to the individual grains"" surface and thus reduces the cleaning effect of the treatment gas.
This invention is accordingly based on the task of specifying a method for the cleaning of SiO2 grain which achieves high grain purity at comparatively little expenditure of time, material and costs, and of providing a simple device suitable for the implementation of the method.
In view of the method, this task is solved on the basis of the initially described cleaning method according to the invention such that SiO2 grain is fed to and heated in a gas stream which is directed towards an impingement plate, the SiO2 grain being accelerated in the direction of the impingement plate such that softened contaminations or melting agglomerates adhere to the impingement plate and cleaned SiO2 grain is removed from the impingement plate.
The SiO2 grain can consist of natural crystalline quartz or of quartz glass grain which in turn may be made of natural quartz or of synthetic starting materials. The contaminations of the SiO2 grain may be either mineral substances such as usually found in natural quartz or substances which were imported into the grain during preparation of the raw materials or in the course of further processing, for example due to abrasion.
SiO2 grain of crystalline quartz melts at approx. 1700xc2x0 C. whereas no defined melting point can be assigned to amorphous SiO2 grain, but much rather a gradual viscosity decrease is observed with increasing temperature. The melting points of purely mineral contaminations or of metallic abrasionsxe2x80x94such as steel for examplexe2x80x94are usually at temperatures of under 1500xc2x0 C. Due to mixing or alloying of the contaminations with substances from the SiO2 grain, the melting or softening temperatures may be even somewhat lower.
The SiO2 grain comprising contaminations is fed to the gas stream and heated therein to a temperature at which the contaminations will soften which is understood to also include melting, or where the contaminations with SiO2 grain form softened melting agglomerates. Hereinafter, the molten or softened contaminations and the melting agglomerates comprising contaminations will be called xe2x80x9csoftened contamination particlesxe2x80x9d. By means of the gas stream, the grain including the softened contamination particles are thrown onto the impingement plate. Since, in the method according to the invention, the grain is heated and fed in the gas stream without contact to the furnace walls, no sticking of the grain to the furnace walls need be expected; and other marginal conditions for the cleaning processes are inapplicable, such as the abrasion resistance or temperature stability of a furnace material. The grain can thus be heated to very high temperatures at which even those contaminations soften, melt up or form melting agglomerates which cannot be removed by the generic method due to their high melting or softening temperatures.
The separation of contaminations from the remaining grain is due to the softened contamination particles or at least part thereof adhering to the impingement plate; however, no pure SiO2 grain will do so, or very little thereof. Decisive for the degree of separation shall be difference between the adhesive capacities of softened contamination particles on the one hand and pure SiO2 grain on the other hand at the impingement plate. The respective adhesive capacities in turn essentially depend on the viscosity immediately before the impingement plate. Best adhesive capacity can generally be expected in a viscosity range of doughy consistency. Ideally, all softened contamination particles will adhere to the impingement plate, but no pure SiO2 grain. Thus, the SiO2 grain is not or only slightly softened in the area of the impingement plate.
After impacting on the impingement plate, the non-adhesive SiO2 grain will be removed from there. The simplest manner is by the force of gravity whereby the SiO2 grain drops to the bottom at a right angle to the impingement plate. However, the impingement plate may also be inclined with regard to the direction of movement of the SiO2 grain so that it does not stop the SiO2 grains"" movement but merely deflects their direction of movement. Especially in the last case, the temporary and loose adhesion of softened contamination particles on the impingement plate may already suffice, if, after adhesion, the further movement of softened contamination particles differs with regard to the velocity and/or direction versus the movement of the SiO2 grain.
Compared with the generic method, the no-contact heating according to the invention will allow the adjustment of higher softening temperatures. Thus, even contaminations with high melting and softening temperatures are obtainedxe2x80x94largely irrespective of the size of the softened contamination particles and melting agglomerates or their mechanical stability. Here, the method according to the invention also enables the removal of contaminations that either cannot be chlorinated at all or barely so.
An impingement plate of quartz glass is advantageously used. Quartz glass excels with its high mechanical resistance and its stability to changing temperatures. Contaminations given off from a quartz glass impingement plate to the SiO2 grain are negligible.
Particularly suitable proved to be an impingement plate whose surface has a mean roughness depth Ra of 0.5 xcexcm or more. Roughness improves the degree of separation by increasing the adhesive capacity for the softened contamination particles while it hardly influences the adhesion of the SiO2 grain which is not or only slightly softened. The value for roughness depth Ra is ascertained according to DIN 4768.
A method is preferred in which the impingement plate is heated. Heating can further improve the degree of separation. During the cleaning process, the impingement plate is kept at a temperature at which the softened contamination particles adhere as optimally as possible to the impingement plate while the SiO2 grain rebounds from it. Heating can also prevent the breaking away of solidifying contamination particles from the impingement plate due to different thermal expansions on both sides.
The impingement plate is advantageously moved perpendicularly to the direction of the gas stream. Thus, fresh impingement plate can be provided as soon as its effect subsides due to the adherent melting agglomerates. The impingement plate can be moved continuously or step-wise. The direction of movement is perpendicular to the direction of the spreading gas stream so that its distance to the impingement plate is equidistant before and after the movement.
It proved advantageous to adjust the temperature of the SiO2 grain to a value in the range of between 1,000xc2x0 C. and 2,200xc2x0 C. in the area of the impingement plate. These temperature data relating to the surface of the grain shall be used only as approximate reference values. Because the grain""s degree of softening not only depends on the temperature but also, for example, on the size of the SiO2 grains and the duration of heating. At high temperatures, even contamination particles with high melting points will soften which are not caught due to the rotary furnace""s limited temperature stability with the generic cleaning method. It is true thatxe2x80x94at temperatures of above approx. 1800xc2x0 C.xe2x80x94superficial melting or softening of SiO2 grain may occur which, however, merely causes a mostly desirable spheroidizing of the SiO2 grain.
The grain""s conglutination is prevented by means of fine dispersion in the gas stream and by the surface""s cooling and sufficient solidification prior to contact with the impingement plate. In the method according to the invention, it is possible to set such high treatment temperatures due to the fact that the grain is heated in the gas stream without having to heat parts of the cleaning device simultaneously high in like manner.
The cleaning effect of the cleaning method according to the invention is even intensified by the SiO2 grain being introduced in fine distribution in the gas stream. Its introduction into the gas stream can be done by spreading or spraying for, example. One benefit with this procedure is that the SiO2 grain is finely distributedxe2x80x94xe2x80x9cdispersedxe2x80x9dxe2x80x94in the gas stream and thus agglomerations of SiO2 grain can be prevented. Such agglomerations are observed with the initially described hot chlorination when the SiO2 grains softened in the rotary furnace begin to adhere to each other. The cleaning effect of the treatment gas is reduced thereby. In contrast, the grains can be separately heated to high temperatures by dispersing themxe2x80x94particularly by spreading or spraying the SiO2 grains in the gas streamxe2x80x94so that they will soften without adhering to one another. Thus, dispersing in the gas stream allows all grains to be exposed uniformly and, moreover, to particularly high temperatures. The cleaning effect will not only be improved thereby, but it is more reproducible because agglomerates are avoided.
The gas stream preferably contains a halogenous treatment gas. The halogenous treatment gas supports the method""s cleaning effect because such contaminationsxe2x80x94forming volatile reaction products with halogensxe2x80x94can be removed even more effectively. Suitable as a halogenous treatment gas are the elements fluorine, chlorine, bromine, iodine, gaseous chemical compounds of these elements and mixtures of the elements and compounds.
One method proved particularly advantageous where the gas stream comprises a burner gas with a hydrogenous component which is burnt by forming a burner gas flame into which the SiO2 grain is continuously fed. Here, the SiO2 grain is exposed to a burner gas flame, heated therein and simultaneously precleaned. The grain is heated by the heat of reaction during combustion of the hydrogenous component of the burner gas. Combustion is understood to mean any form of exothermic fast oxidation of the hydrogenous component which enables the grain""s heating. Suitable as the hydrogenous component are for example hydrogen or hydrocarbon compounds such as propane or acetylene. As the reaction partner for the combustion of the hydrogenous component may be used for example oxygen, oxygen compounds, halogens and halogen compounds. A halogenous component of the burner gas will achieve an additional cleaning effect. By means of the burner gas flame, higher temperatures may be set, and fast temperature changes are allowed. Accordingly, the SiO2 grain can be exposed to very fast temperature changes in the burner gas flame and thus be heated abruptly. SiO2 grains may burst thereby, primarily grains with interior stresses due to prior damage and structural defects. Cause for such defects are frequently foreign atoms. Due to the bursting grains, foreign atoms come to the free surface from where they can be easily removed by softening and separation by means of adhesion to the impingement plate as well asxe2x80x94favored by the high temperaturesxe2x80x94by reaction with halogenous components of the gas stream.
The method according to the invention enables continuous cleaning because the SiO2 grains are continuously fed to the gas stream.
It proved advantageous to feed heated SiO2 grains to the gas stream. This will reduce the time span required for softening the contaminations.
For a solution of the above specified task with regard to the device for the implementation of the method, the device is provided with a burner according to the invention into which a burner gas is introduced by means of which a burner gas flame is produced, and with a feeder device by means of which the SiO2 grain containing contaminations is fed to the burner gas flame, and with an impingement plate toward which the burner gas flame is directed.
The burner gas flame produced by means:.of the burner serves to heat the SiO2 grain. To this end, the SiO2 grain containing the contaminations is fed to the burner gas flame by means of the feeder device and, therein, it is heated to a temperature at which the contaminations are forming softened contamination particles. The SiO2 grain can be fed to the burner gas flame through the burner. If necessary, the burner is connected with the feeder device, and has a corresponding burner nozzle for the SiO2 grain. Yet, feeding the grains. may also comprise a spreading with the feeder device then formed as a chute device. The SiO2 grains may also be fed in by spraying or injection; in these cases, the feeder device comprises a pressure device for producing high pressure or low pressure, and it further comprises a nozzle. It is essential that, by means of the feeder device, the SiO2 grain can be fed to the burner gas flame and be dispersed therein.
Moreover, the device according to the invention is characterized by an impingement plate which is directed to the burner gas flame. In the simplest case, the impingement plate is planar in form. However, it may also have other forms; for example, the impingement plate may be arched or provided with surface structures. By means of the burner gas flame, the grain is heated without contact to any furnace walls and subsequently thrown onto the impingement plate together with the softened contamination particles. Thus, adhesion of grains to the furnace walls need not be expected; and marginal conditions are inapplicable, such as the abrasion resistance or the temperature stability of a furnace material. Thus, the device according to the invention may be designed especially simple. The grain is heated to very high temperatures without any contact, with even those contaminations softening, melting or forming melting agglomerates which cannot be removed due to their high melting or softening temperatures by means of the generic device.
Reference is made to the above explanations on the method according to the invention, with regard to the separation of,contaminations from the remaining grains by adhesion to the impingement plate, its effect and its arrangement relative to the direction of movement of the SiO2 grain.
The impingement plate is advantageously made of quartz glass. Quartz glass is characterized by high mechanical strength and its stability with regard to temperature changes. Contaminations given off from the quartz glass impingement plate due to abrasion are negligible.
An impingement plate proved especially suitable whose surface has a mean roughness depth Ra of 0.5 xcexcm or more. Roughness improves the degree of separation by increasing the adhesive capacity for the softened contamination particles while it hardly influences the adhesion of the SiO2 grain which is not softened at all or only slightly softened.
An impingement plate proved favorable whose normal surface is inclined relative to the direction of the burner gas flame or is running at an oblique angle. Such alignment of the impingement plate relative to the spreading direction of the burner gas flame and thus also relative to the direction of movement of the SiO2 grain may influence the separation of contaminations from SiO2 grains. In the simplest case, the SiO2 grain impacts the impingement plate at a right angle and will be thereafter removed from the impingement plate as the SiO2 grain drops to the bottom, for example in a collection vessel arranged underneath the impingement plate. Here, contaminations dropping from the impingement plate may also fall into the collection vessel. This can be prevented by an inclined impingement plate where the SiO2 grain""s movement is not stopped but its direction of movement is deflected so that the grains are collected at a point where no contaminations dropping from the impingement plate may arrive. Moreover, the inclination of the impingement plate relative to the SiO2 grain""s direction of movement may influence the adhesive properties of the SiO2 grain and the softened contamination particles.
Preferable is a variation of the device according to the invention where the impingement plate is connected with a temperature facility. By means of the temperature facility, the impingement plate can be heated to a nominal temperature, or cooled down to it, or maintained at the nominal temperature.
Regarding the implementation of an inclination of the impingement plate relative to the SiO2 grain""s direction of movement and the advantages resulting therefrom, reference is made to the above explanations for the method according to the invention. An impingement plate proved advantageous which is defined by an orthogonal which is inclined relative to the direction of the gas stream or is running at an oblique angle. In a simple embodiment of the device according to the invention, the impingement plate is tiltably designed so that the tilting angle can easily be adjusted and optimized with regard to the separation and the adhesive properties.
Particularly simple in design is an embodiment of the device according to the invention where a chlorine/oxyhydrogen burner is used as a burner. High temperatures of more than 1300xc2x0 C. can easily be achieved by means of the chlorine/oxyhydrogen flame. Moreover, the chlorine will contribute to the cleaning effect as already explained above in more detail on the basis of the method according to the invention.
In view of the avoidance of contaminations due to abrasion or chemical stripping, burner and/or feeder device will advantageously consist of quartz glass.
It proved favorable to provide the feeder device with a heating apparatus acting upon the SiO2 grain. By means of the heating apparatus, the SiO2 grain can be preheated which can reduce the required dwell time of the grain in the burner gas flame. This is especially of advantage if the distance between burner gas flame and impingement plate is short.